Fabrication II - Deposition-1

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II. Thin Film Deposition
Physical Vapor Deposition (PVD)
- Film is formed by atoms directly transported from source to the substrate
through gas phase
• Evaporation
• Thermal evaporation «
• E-beam evaporation «
• Sputtering
• DC sputtering
«
• DC Magnetron sputtering
«
• RF sputtering
«
• Reactive PVD
Chemical Vapor Deposition (CVD)
- Film is formed by chemical reaction on the surface of substrate
• Low-Pressure CVD (LPCVD)
«
• Plasma-Enhanced CVD (PECVD)
«
• Atmosphere-Pressure CVD (APCVD)
• Metal-Organic CVD (MOCVD)
Oxidation
Spin Coating
Platting
Applied Physics 298r
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E. Chen (4-12-2004)
General Characteristics of Thin Film Deposition
• Deposition Rate
• Film Uniformity
• Across wafer uniformity
• Run-to-run uniformity
• Materials that can be deposited
• Metal
• Dielectric
• Polymer
• Quality of Film – Physical and Chemical Properties
• Stress
• Adhesion
• Stoichiometry
• Film density, pinhole density
• Grain size, boundary property, and orientation
• Breakdown voltage
• Impurity level
• Deposition Directionality
• Directional: good for lift-off, trench filling
• Non-directional: good for step coverage
• Cost of ownership and operation
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Evaporation
¨ Load the source material-to-bedeposited (evaporant) into the
container (crucible)
¨ Heat the source to high
temperature
¨ Source material evaporates
¨ Evaporant vapor transports to and
Impinges on the surface of the
substrate
¨ Evaporant condenses on and is
adsorbed by the surface
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Substrate
Film
Evaporant
Vapor
Current
Crucible (energy source)
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Langmuire-Knudsen Relation
Mass Deposition Rate per unit area of source surface:
Substrate
1
2
1
M 
Rm = Cm   cos θ cos ϕ 2 (Pe (T ) − P )
r
T 
r
θ
Cm = 1.85x10-2
r:
source-substrate distance (cm)
T: source temperature (K)
Pe: evaporant vapor pressure (torr), function of T
P: chamber pressure (torr)
M: evaporant gram-molecular mass (g)
¬ Maximum deposition rate reaches at high
chamber vacuum (P ~ 0)
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ϕ
P
Pe
Source (K-Cell)
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Uniform Coating
Spherical surface with source on its edge:
Spherical Surface
r
cos θ = cos ϕ =
2r0
ϕ
1
2
 M  Pe
Rm = Cm  
2
 T  4r0
r0 θ
P
¨ Angle Independent – uniform coating!
¬ Used to coat instruments with spherical surfaces
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Pe
Source (K-Cell)
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Uniformity on a Flat Surface
Consider the deposition rate difference
between wafer center and edge:
R1 ∝
W /2
1
2
r1
2
1
r
R2 ∝ 2 cos 2 θ = 1 4
r2
r2
ϕ
r1
θ r2
Define Uniformity:
σ (% ) =
P
R1 − R2
(% )
R1
Pe
−2
2
  W 2 
W
σ = 1 − 1 +    ≈ 2
  2r1  
2r1


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or
W
= 2σ
r1
6
Source (K-Cell)
E. Chen (4-12-2004)
Wafer
Uniformity Requirement on a Flat Surface
Source-substrate distance requirement:
W
2σ
In practice, it is typical to double this
number to give some process margin:
r >W
2
σ
Source-Sample Distance (r)
r>
160
Larger r Means:
¬ bigger chamber
¬ higher capacity vacuum pump
¬ lower deposition rate
¬ higher evaporant waste
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1%
2%
120
5%
100
10%
80
60
40
20
0
0
2
4
6
8
Sample Size (W)
Another Common Solution:
off-axis rotation of the sample
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Thickness Deposition Rate vs. Source Vapor Pressure
dh Rm
=
Ae
dt
ρ
Thickness deposition rate
Substrate
Film
dh
1
2
dh Ae
1
M 
= Cm   cos θ cos ϕ 2 Pe (T )
dt
ρ
r
T 
T:
Ae:
ρ:
θ
source temperature (K)
source surface area (cm2)
evaporant density (g/cm3)
Ae
Pe is function of source Temperature!
(A/s)
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¬
ϕ
P
Pe
T
Example: Al
M ~ 27, ρ ~ 2.7, Ae ~ 10-2 cm2, T ~ 900 K
R ~ 50 cm (uniformity requirement)
dh
= 50 Pe
dt
r
Source (K-Cell)
The higher the vapor pressure, the higher the material’s
deposition rate
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Deposition Rate vs. Source Temperature
Typically for different material:
dh
= (10 ~ 100) Pe (T )
dt
•
•
•
•
( A / s)
For deposition rate > 1 A/s:
Pe > ~ 100 mtorr
Pe depends on: 1) materila
and 2) temperature
Deposition rates are
significantly different for
different materials
Hard to deposit multicomponent (alloy) film
without losing stoichiometry
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Example: for Pe > 100 mtoor
T(Al) > 1400K, T(Ta) > 2500K
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Heating Method – Thermal (Resist Heater)
Source
Material
Resistive
Wire
Current
Foil Dimple Boat
Crucible
Alumina Coated
Foil Dimple Boat
Contamination Problem
with Thermal Evaporation
Container material also evaporates, which
contaminates the deposited film
Cr Coated Tungsten Rod
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CIMS’ Sharon Thermal Evaporator
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Heating Method – e-Beam Heater
e-
Electron Beam
Crucible
Magnetic Field
Evaporant
(beam focusing
& positioning)
Evaporant
Focusing
Aperture
Cathode Filament
Water Cooled Rotary Copper Hearth
(Sequential Deposition)
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Advantage of E-Beam Evaporation:
Very low container contamination
E. Chen (4-12-2004)
CIMS’ Sharon E-Beam Evaporator
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Comparison
Deposition
Thermal
E-Beam
•
•
Material
Typical
Evaporant
Impurity
Deposition
Rate
Temperature
Range
Cost
Metal or low
melt-point
materials
Au, Ag, Al, Cr, Sn,
Sb, Ge, In, Mg,
Ga
CdS, PbS, CdSe,
NaCl, KCl, AgCl,
MgF2, CaF2, PbCl2
High
1 ~ 20 A/s
~ 1800 ºC
Low
Both metal
and
dielectrics
Everything above,
plus:
Ni, Pt, Ir, Rh, Ti,
V, Zr, W, Ta, Mo
Al2O3, SiO, SiO2,
SnO2, TiO2, ZrO2
Low
10 ~ 100
A/s
~ 3000 ºC
High
Stoichiometrical Problem of Evaporation
Compound material breaks down at high temperature
Each component has different vapor pressure, therefore different deposition
rate, resulting in a film with different stoichiometry compared to the source
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Typical Boat/Crucible Material
Refractory Metals
Material
Melting Point
(ºC)
Temperature for 10-mtorr
Vapor Pressure (Pe)
(ºC)
Tungsten (W)
3380
3230
Tantalum (Ta)
3000
3060
Molybdenum (Mo)
2620
2530
Refractory Ceramics
Graphitic Carbon (C)
3799
2600
Alumina (Al2O3)
2030
1900
Boron Nitride (BN)
2500
1600
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DC Diode Sputtering Deposition
2 – 5kV
• Target (source) and substrate are placed
on two parallel electrodes (diode)
• They are placed inside a chamber filled
with inert gas (Ar)
• DC voltage (~ kV) is applied to the diode
Target (Cathode)
• Free electron in the chamber are
e-
e-
accelerated by the e-field
• These energetic free electrons inelastically
Ar
γ
Ar+
e- Ar
collide with Ar atoms
’ excitation of Ar ¨ gas glows
Substrate (Anode)
’ ionization of Ar ¨ Ar+ + 2nd electron
• 2nd electrons repeat above process
¨ “gas breakdown”
¨ discharge glow (plasma)
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Self-Sustained Discharge
• Near the cathode, electrons move much faster than ions
because of smaller mass
¬ positive charge build up near the cathode, raising
the potential of plasma
¬ less electrons collide with Ar
¬ few collision with these high energetic electrons
results in mostly ionization, rather than excitation
¬ dark zone (Crookes Dark Space)
• Discharge causes voltage between the electrodes
reduced from ~103 V to ~102V, mainly across the dark
space
• Electrical field in other area is significantly reduced by
screening effect of the position charge in front of
cathode
• Positive ions entering the dark space are accelerated
toward the cathode (target), bombarding (sputtering) the
target
¬ atoms locked out from the target transport to the
substrate (momentum transfer, not evaporation!)
¬ generate 2nd electrons that sustains the discharge
(plasma)
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2 – 5kV
Target (Cathode)
+ + + + +
Ar+
Crookes
Dark Space
Ar+
t
t
Substrate (Anode)
E. Chen (4-12-2004)
Requirement for Self-Sustained Discharge
• If the cathode-anode space (L) is less than the dark space length
¬
ionization, few excitation
¬
cannot sustain discharge
• On the other hand, if the Ar pressure in the chamber is too low
¬
Large electron mean-free path
¬
2nd electrons reach anode before colliding with Ar atoms
¬
cannot sustain discharge either
L ⋅ P > 0.5 (cm ⋅ torr )
Condition for Sustain Plasma:
L: electrode spacing, P: chamber pressure
For example:
Typical target-substrate spacing: L ~ 10cm
¨ P > 50 mtorr
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Deposition Rate vs. Chamber Pressure
High chamber pressure results in low deposition rate
Mean-free path of an atom in a gas ambient:
In fact, sputtering deposition rate R:
5 × 10 −3
λ~
(cm)
P (torr )
R∝
Use previous example:
L = 10 cm, P = 50 mtorr
¨ λ ~ 0.1 cm
¨ sputtered atoms have to go through
hundreds of collisions before reaching the
substrate
¨ significantly reduces deposition rate
¨ also causes source to deposit on chamber
wall and redeposit back to the target
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L⋅P
’ Large LP to sustain plasma
’ small LP to maintain good
deposition rate and reduce
random scattering
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?
DC Magnetron Sputtering
• Using low chamber pressure to maintain high deposition rate
• Using magnetic field to confine electrons near the target to sustain plasma
E
e-
+ B
+
S
Cathode (Target)
N
S
Apply magnetic field parallel to the
cathode surface
Target
¨ electrons will hope (cycloid) near the
S
surface (trapped)
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N
E. Chen (4-12-2004)
S
Impact of Magnetic Field on Ions
Hoping radius r:
r~
1
B
2m
Vd
e
Ar+
Vd – voltage drop across dark space
(~ 100 V)
B – Magnetic field (~ 100 G)
e-
E
+ B
r
Cathode (Target)
For electron
r ~ 0.3 cm
For Ar+ ion:
r ~ 81 cm
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As A Result …
¬ current density (proportional to ionization rate) increases by 100 times
¬ required discharge pressure drops 100 times
¬ deposition rate increases 100 times
Deposition Rate (R)
Magnetron
Non-Magnetron
~ 1mT
~ 100mT
Chamber Pressure (P)
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RF (Radio Frequency) Sputtering
DC sputtering cannot be used for depositing
dielectrics because insulating cathode will cause
charge build up during Ar+ bombarding
13.56 MHz
~
¨ reduce the voltage between electrodes
¨ discharge distinguishes
Target
Solution: use AC power
• at low frequency (< 100 KHz), both electrons and
ions can follow the switching of the voltage –
¨ DC sputtering
• at high frequency (> 1 MHz), heave ions cannot no
long follow the switching
¨ ions are accelerated by dark-space (sheath)
voltage
¨ electron neutralizes the positive charge buildup on
both electrodes
• However, there are two dark spaces
¨ sputter both target and substrate at different cycle
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Target Sheath
e-
eAr
Ar+
t
Substrate Sheath
Substrate
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RF (Radio Frequency) Sputtering
13.56 MHz
A 
VT
∝  S 
VS
 AT 
VT
Vs
AT
As
–
–
–
–
n
~
(n ~ 2)
AT
voltage across target sheath
voltage across substrate sheath
area of target electrode
area of substrate electrode
Target
VT
AS
VS
Substrate
Larger dark-space voltage develops at the
electrode with smaller area
¨ make target electrode small
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Comparison between Evaporation and Sputtering
Evaporation
Low energy atoms
(~ 0.1 eV)
Sputtering
High energy atoms / ions
(1 – 10 eV)
• denser film
• smaller grain size
• better adhesion
High Vacuum
• directional, good for lift-off
• lower impurity
Low Vacuum
• poor directionality, better step coverage
• gas atom implanted in the film
Point Source
• poor uniformity
Parallel Plate Source
• better uniformity
Component Evaporate at Different Rate
• poor stoichiometry
All Component Sputtered with Similar Rate
• maintain stoichiometry
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Chemical Vapor Deposition (CVD)
Deposit film through chemical reaction and surface absorption
• Introduce reactive gases to the chamber
• Activate gases (decomposition)
A B
¬ heat
A
B
¬ plasma
• Gas absorption by substrate surface
B
A
• Reaction take place on substrate surface;
A
W
film firmed
B
W
Substrate
• Transport of volatile byproducts away
form substrate
• Exhaust waste
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Types of CVD Reactions
Pyrolysis (Thermal Decomposition)
AB ( gas ) → A ( solid ) + B ( gas )
Example
α-Si deposited at 580 - 650 ºC:
SiH 4 ( gas ) = Si ( solid ) + 2 H 2 ( gas )
Reduction (lower temperature than Pyrolysis)
AB ( gas ) + H 2 ( gas, commonly used ) ↔ A ( solid ) + HB ( gas )
Example
W deposited at 300 ºC:
WF6 ( gas ) + 3H 2 ( gas ) = W ( solid ) + 6 HF ( gas )
Reversible process, can be used for chamber cleaning
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Types of CVD Reactions (Cont.)
Oxidation
AB ( gas or solid ) + O2 ( gas, commonly used ) ↔ AO ( solid ) + [O ]B ( gas )
Example
Low-temperature SiO2 deposited at 450 ºC:
SiH 4 ( gas ) + O2 ( gas ) = SiO2 ( solid ) + 2 H 2 ( gas )
Example
SiO2 formed through dry oxidation at 900 - 1100 ºC:
Si ( Solid ) + O2 ( gas ) = SiO2 ( solid )
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Types of CVD Reactions (Cont.)
Compound Formation
AB ( gas or solid ) + XY ( gas or solid ) ↔ AX ( solid ) + BY ( gas )
Example
SiO2 formed through wet oxidation at 900 - 1100 ºC:
Si ( Solid ) + 2 H 2O(vapor ) = SiO2 ( solid ) + 2 H 2
Example
SiO2 formed through PECVD at 200 - 400 ºC:
Si H 4 ( gas ) + 2 N 2O( gas ) = SiO2 ( solid ) + 2 N 2 + 2 H 2
Example
Si3N4 formed through LPCVD at 700 - 800 ºC:
3Si H 2Cl2 ( gas ) + 4 NH 3 ( gas ) = Si3 N 4 ( solid ) + 6 H 2 + 6 HCl
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CVD Deposition Condition
Mass-Transport Limited Deposition
- At high temperature such that the reaction rate
- Gas delivering controls film deposition rate
- Film growth rate insensitive to temperature
- Film uniformity depends on whether reactant
can be uniformly delivered across a wafer and
wafer-to-wafer
Reaction-Rate Limited Deposition
Deposition Rate (log)
exceeds the gas delivering rate
Mass-Transport
Limited
Regime
Reaction-Rate
Limited
Regime
- At low temperature or high vacuum such that
1/T (K)
the reaction rate is below gas arriving rate
- Temperature controls film deposition rate
- Film uniformity depends on temperature
uniformity across a wafer and wafer-to-wafer
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Low-Pressure CVD (LPCVD)
Heater
Heater
Heater
Reactant
Gas
Horizontal
Quartz Tube
Exhausted
Gas
Wafer
Z-1
Z-2
Z-3
• Thermal energy for reaction activation
• System works at vacuum (~ 0.1 – 1.0 torr), resulting in high diffusivity of reactants
¨ reaction-rate limited
• Wafer can stacked closely without lose uniformity as long as they have the same
temperature
• Temperature is controlled around 600 - 900ºC by “flat” temperature zone through using
multiple heaters
• Low gas pressure reduce gas-phase reaction which causes particle cluster that
contaminants the wafer and system
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Plasma-Enhanced CVD (PECVD)
RF
• Use rf-induced plasma (as in sputtering
~
Gases
case) to transfer energy into the reactant
gases, forming radicals (decomposition)
• Low temperature process (< 300 ºC)
• For depositing film on metals and other
materials that cannot sustain high
eA
temperature
Shaw Heads
B
e-
eA+
B+
e-
• Surface reaction limited deposition;
substrate temperature control (typically
Substrate
cooling) is important to ensure uniformity
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Common CVD Reactants
Material
LPCVD
PECVD
α-Si
SiH4
SiH4
SiH2Cl2
SiO2
Si(OC2H5)4 (TEOS)
SiH2Cl2 + N2O
SiH4 + N2O
SiH4 + O2
Si3N4
SiH4 + NH3
SH2Cl2 + NH3
SiH4 + NH3
SiH4 + N2
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Comparison of Typical Thin Film Deposition Technology
Process
Material
Uniformity
Impurity
Grain Size
Film
Density
Deposition
Rate
Substrate
Temperature
Directional
Cost
Thermal
Evaporation
Metal or
low
meltingpoint
materials
Poor
High
10 ~ 100
nm
Poor
1 ~ 20 A/s
50 ~ 100 ºC
Yes
Very low
Poor
Low
10 ~ 100
nm
Poor
10 ~ 100 A/s
50 ~ 100 ºC
Yes
High
~ 200 ºC
Some
degree
High
E-beam
Evaporation
Both metal
and
dielectrics
Sputtering
Both metal
and
dielectrics
Very good
Low
~ 10 nm
Good
Metal:
~ 100 A/s
Dielectric:
~ 1-10 A/s
PECVD
Mainly
Dielectrics
Good
Very low
10 ~ 100
nm
Good
10 - 100 A/s
200 ~ 300 ºC
Some
degree
Very High
LPCVD
Mainly
Dielectrics
Very Good
Very low
1 ~ 10 nm
Excellent
10 - 100 A/s
600 ~ 1200 ºC
Isotropic
Very High
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